Diagnosis of acute renal failure

This website was awesome and has all these easy-to-read tables on how to diagnose acute renal failure:
http://www.aafp.org/afp/20000401/2077.html

The difference between acute and chronic renal failure
- Acute renal failure is a deterioration in kidney function over a period of days or weeks, which is usually reversible
- Chronic renal failure is long-standing and progressive impairment of renal function, which is usually irreversible

Three types of renal failure:
• Prerenal – diminished renal blood flow
• Intrinsic – damage to renal prenchyma
• Postrenal – urinary tract obstruction

How to diagnose:
1. Thorough history and physical examination
o Symptoms
 Anorexia
 Fatigue
 Mental status changes
 Nausea and vomiting
 Pruritus (itching)
 Seizures (if blood urea nitrogen level is very high)
 Shortness of breath (if volume overload is present)

o Physical findings
 Asterixis (flapping tremor)and myoclonus (sudden twitch of muscle)
 Pericardial or pleural rub
 Peripheral edema (if volume overload is present)
 Pulmonary rales/crackles (if volume overload is present)
 Elevated right atrial pressure (if volume overload is present)

2. Blood and urine tests
• Blood test
o BUN (blood urea nitrogen)
o Serum electrolyte
o Creatinine
o Calcium
o Phosphorus
o Albumin levels
o Complete blood count
• Urine test
o Dipstick test
o Microscopy
o Sodium
o Creatinine levels
o Urine osmolarity

3. If necessary, a renal biopsy may need to be performed if there is intrinsic renal failure that is not acute tubular necrosis.
o Complications (very low chance, less that 1%) – bleeding, arteriovenous fistula, death

Prerenal failure (problem = impaired renal blood flow – intravascular depletion, decreased effective circulating volume to kidneys or agents that impair renal blood flow):
• Urine and blood studies
o Few hyaline casts
o Urine osmolarity of greater than 500 mOsm [normal = 50-1400 mOsm/kg)
o Fraction of excreted sodium – less than 1% (kidneys respond as if volume depletion has occurred – absorb sodium to absorb water)
o Parenchyma is undamaged

Intrinsic kidney failure (injury to renal parenchyma):
• Impaired sodium reabsorption (due to parenchymal damage)
o sodium fraction excreted is greater than 3%
• Isotonic urine osmolarity – 250-300 mOsm

Postrenal acute renal failure (outflow tract of kidney/s is blocked):
• Severe oligonuria (small urine volume) or anuria
o Output less than 100 mL per day (normal = 1-2L)